The present invention relates to a heat exchanger made of aluminum and provided with fins which have a hydrophilic coating.
The term "aluminum" as used herein and in the appended claims includes pure aluminum, commercial aluminum containing small amounts of impurities, and aluminum alloys consisting predominantly of aluminum.
Generally with heat exchangers, especially with the evaporators of air conditioning apparatus, the surface temperature of the fins on the tubes falls below the dew point of the atmosphere, so that drops of water adhere to the surfaces of the fins. The deposition of such water drops results in increased resistance to the flow of air, reducing the amount of flow of air and entailing a decreased heat exchange efficiency. This tendency becomes more pronounced when the spacing between the fins is reduced to improve the performance of the heat exchanger and to diminish the size thereof. The heat exchange efficiency is greatly influenced by the wettability of the fin surface with water. When the fin surface has good wettability, the water deposited thereon is less likely to become water drops. This results in reduced resistance to the flow of air and an increased amount of flow of air to achieve a higher heat exchange efficiency. To give improved wettability to fin surfaces, a process has been proposed for forming a coating of water glass (alkali silicate) on the surfaces of aluminum fins (see Published Examined Japanese Patent Application No. 48177/1978). This gives an improved hydrophilic property to the fin initially, but the hydrophilic property becomes impaired early and is not sustainable satisfactorily. Further when the fin material to be shaped is formed with the water glass coating which is hard, cracks develop in bent portions of fins when the material is burred to form the fins, hence poor shapability. The coated material, moreover, is liable to cause wear on the die.